8

Fuel Conversion Efficiency and Other Material Driven Opportunities

Although not directly related to the sourcing, storage, or transmission of energy, maximizing the utility of each megawatt-hour of energy delivered to the field is important to enable increased self-sustainability. This awareness minimizes the amount of energy that must be transported to the battlefield or collected locally.

To accomplish this, fuel-conversion efficiency needs to be maximized throughout the complete chain from energy storage to power delivery. For example, lower rolling-resistance tracks, higher temperature–capable power electronics, batteries, motors, and more-efficient cooling systems, together could enable considerable reductions in parasitic cooling and friction losses. Some of these opportunities are described below.

PRESENT ARMY POWER PACK FUEL EFFICIENCY AND PERFORMANCE UPGRADES

The Army already has a number of active power pack initiatives in this area, which are then balanced against other key performance objectives such as power density and heat rejection. These initiatives are summarized below.

The Advanced Powertrain Demonstrator (APD) power pack presently under development includes the following: (1) a low heat–rejection, high-efficiency, two-stroke opposed-piston engine, (2) a wide range, high-efficiency cross-drive transmission, (3) an advanced cooling/thermal management system, and (4) an advanced high-efficiency inline starter generator. Due to its increase in power density, it enables increased terrain

access and higher vehicle speed power packs using military on-the-shelf (MOTS) components (see Figure 8.1).

The “representative area of interest” terrain maps in Figure 8.2 show results from modeling the performance of the present Bradley fighting vehicle against that of a future Bradley fighting vehicle that includes the

APD power pack. Whereas the present Bradley cannot traverse 22 percent of the terrain, the future Bradley can traverse all but 6 percent of the terrain. This added capability is essential, because without it, combatants can predict the path of the Bradley, making it easier for them to set up their defenses. Also shown above, the Bradley’s average velocity across the best 50 percent of this terrain increases with the APD power pack from 10 to 15 mph.

The Advanced Combat Engine (ACE), part of the APD, is a 746-kW four-cylinder, two-stroke compression ignition engine with horizontally opposed pistons (see Figure 8.3). As a two stroke (firing every two strokes versus every four strokes for more conventional engines), the ACE provides the capability for higher power per unit of displacement. In a horizontally opposed piston engine, there is no cylinder head. Instead, opposed pistons approach one another as they are moving to their minimum displacement position.

Without a cylinder head (unlike a conventional diesel), no heat is transferred into the head. This effect results in reduced engine heat rejection, particularly important because armored ground vehicles with their constrained grille open area pay a huge penalty for cooling system losses.

The Advanced Combat Transmission (ACT), part of the APD, is a high efficiency, drive-by-wire transmission, which replaces traditional, inefficient mechanisms like hydraulic pumps in the propulsion and steering

systems with solenoid electromagnetic controls. The steer-by-wire system is claimed to provide optimal control of the vehicle at high speed as well as during sharp turns. Lastly, it has an unusually high number of forward gear ratios providing a wide ratio range enabling the engine to operate at its most efficient speed/load point for a given power demand. Whereas some transmissions in Army platforms have efficiencies as low as 55 percent depending on the operating range, SAPA Transmission’s ACT1000 transmission efficiency (output shaft power divided by input shaft power) exceeds 90 percent in every operating condition.¹

The Advanced Thermal Management System (ATMS), part of the APD and under development by AVL, provides the necessary power plant cooling system. It replaces traditional filters, which wear out in 20 hours in dusty areas like deserts, with a pulse-jet air cleaner that cleans itself with short-duration pulses of compressed air. This redesign results in additional air flow and is projected to last a minimum of 500 h.²

The APD Combat Vehicle Integrated Starter Generator (ISG), part of the APD, produces 160 kW, many times more than what is currently available on the Bradley from its present alternator off the engine. It will not require its own dedicated cooling system, because it can function using a common 105°C coolant with the engine block. Internally, silicon carbide power-electronic devices are used because they have an operating temperature limit of 200°C, which compares with the roughly 125°C limit for silicon. The required heat-management system (i.e., the heat sinking) therefore can be smaller with silicon carbide devices when both are maintained at the same case (package) temperatures of 105°C.³ Aggressive targets for these APD powertrain technologies in 2035 and 2050 already have been established by the Army Ground Vehicle Systems Center.

Another advanced propulsion system presently being defined by the Army Ground Vehicle Systems Center is simply entitled the “Projected Propulsion System.” This hybrid power pack includes the following: (1) a high-efficiency, fuel conversion source (engine or fuel cell), (2) a high-efficiency power/torque conversion device, (3) variable speed fan drive, (4) an 80-kWh energy storage device enabling idle engine shutoff and silent mobility, and (5) highly efficient battery charging.⁴

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¹ SAPA Transmission, “ACT 1000 Transmission,” https://sapatransmission.com/products/act-1000-transmission/, accessed November 2020.

² U.S. Army CCDC Ground Vehicle Systems Center (formerly TARDEC), 2015, “TARDEC 30-Year Strategy Value Stream Analysis,” U.S. Army, https://api.army.mil/e2/c/downloads/405983.pdf.

³ S. Freedberg, 2019, “Army Revs Up High-Tech Tank Engine,” Breaking Defense, https://breakingdefense.com/2019/12/army-revs-high-tech-tank-engine/.

⁴ P. Schihl, U.S. Army CCDC Ground Vehicle Systems Center, 2020, “Combat Ground Vehicle Propulsion Efficiency Discussion,” presentation to the committee on April 7 and email provided to individual committee member.

Another program is the Advanced Mobility Experimental Prototype (AMEP), which is to demonstrate potential propulsion solutions for the Extended Range Cannon Artillery program, a self-propelled howitzer. This prototype likely will use selected portions of the APD power pack and include an advanced lower rolling–resistance track and a 150 kWe integrated starter generator. Spanning fiscal years (FYs) 2020 through 2023, 6.3 funding of $16.5 million is approved with an additional $34.9 million funding anticipated.⁵

Still another program is the Platform Electrification Mobility Demonstrator. This program will include multiple vehicle prototype builds to demonstrate electrification capability in tracked combat applications. It will include 15–30 ton light and 35–60 ton heavy ground combat vehicles using a modular approach. The focus will be on hybrid electric propulsion system configurations. Spanning FY 2020 through 2025, 6.2/6.3 funding of $219 million is anticipated.⁶ M2 Bradley and M113 armored personnel carriers will be used as the base platforms.

Key elements of the study include the following: (1) high-temperature power electronics, motors, and generators and (2) investigation of fuel-cell capability to recharge batteries for on-board electric power, silent-mobility capability with an 80 kWh battery pack target for the heavy variant. Transmission alternatives to be evaluated include a cross-drive system (which integrates braking and motoring and enables one track to run at a higher speed than the other for steering) and independent track drives.

FURTHER EFFICIENCY IMPROVEMENTS IN COMPRESSION IGNITION ENGINES

Within the last decade, there have been some very impressive improvements in the efficiency and power density of compression ignition engines, in large part driven by the SuperTruck projects undertaken by Cummins, Navistar, Daimler, Volvo, and PACCAR. Base engine thermal efficiencies exceeding 50 percent at their best speed/load operating point have been demonstrated.⁷

The brake thermal efficiency (BTE) of present four-stroke engines in Army service, such as High Mobility Multipurpose Wheeled Vehicles (HMMWVs), Bradleys, and Strykers, typically range from the high 30s to low 40s. Modernization of Army engine hardware to commercial BTE levels (approaching 50 percent) would reduce jet propellant 8 (JP8) usage by roughly 20 percent. This decrease combined with the use of DF2 diesel

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⁵ Ibid.

⁶ Ibid.

⁷ A summary of the design actions taken on SuperTruck projects is included in Appendix J.

fuel with its 9 percent higher energy content by volume, and further improvements made possible by adjusting injection timing/quantity, could reduce total fuel transported to the battlefield by almost a third.⁸

In addition to base engine improvements, the SuperTruck projects have also included demonstration of various waste-heat recovery systems (see Appendix J). If containable within the space constraints of new ground combat vehicles, they offer a 3 to 5 percent opportunity to reduce fuel use further, thereby increasing the vehicle range and shortening the fuel supply line. Most of the SuperTruck programs focus on the organic Rankine cycle (using cyclopentane). Encouraging work at Southwest Research Institute focusing on the Brayton cycle (using supercritical CO₂) offers the potential for even further efficiency gains. Department of Energy (DOE) SuperTruck advances, including waste-heat recovery concepts, could be leveraged for military applications and provide the potential to significantly improve vehicle range and reduce the JP8 supply line.

Also included in Appendix J is a list of the possible design/development actions that might be considered on future horizontally opposed two-stroke compression ignition engine designs to enable some of the aggressive targets in these areas that the Army is setting for 2035 and beyond while maintaining low heat rejection.

THERMAL BARRIER COATINGS

Reduced heat rejection from a ground combat vehicle’s power plant is critically important. Unlike commercial and light-duty diesel trucks, a combat vehicle’s grille open area needs to be minimized to minimize its susceptibility to enemy projectiles. Lower heat-rejection values also reduce the vehicle’s thermal signature. Lastly, heat not lost in the cooling system can power the vehicle’s propulsion, providing improved fuel economy and range.

For these reasons, thermal barrier coatings (TBCs) of engine components (pistons, cylinder heads, valves) have been a highly desirable study area for many years dating back to 1950s adiabatic engine studies (so called because in theory heat would neither enter nor leave the system). Managing heat flows throughout the power cylinder have always proven to be critically important, as the engine power cylinder surfaces are exposed to flame and extremely high pressure.

Historically, adhesion of ceramic-based thermal barrier coatings has proven to be a major inhibitor to getting thermal barrier coatings into production. Thin coatings adhered but did not provide a significant decrease

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⁸ U.S. Army CCDC Ground Vehicle Systems Center, 2020, verbal communication with committee member.

in thermal conductivity. Thicker coatings provided the needed decrease in thermal conductivity but presented adhesion problems over time. More recently, it has been discovered that a functional coating also must have low thermal conductivity, excellent adhesion, and a low specific heat capacity. Without this low specific heat, the surfaces remain hot, compromising the volumetric efficiency (the engine’s ability to ingest air).

Toyota has been the clear leader in this technology, having introduced their “thermo swing wall insulation” into production in 2015. This SiRPA (a silica-reinforced porous, anodized aluminum) coating, used on aluminum pistons, is claimed to reduce the cooling loss during combustion by about 30 percent.⁹

To deal with higher peak–cylinder pressures and temperatures, newer heavy-duty diesel engines are using steel pistons in lieu of aluminum. While several different original equipment manufacturer (OEM) component suppliers, coating suppliers, and universities are developing their own formulations for these, there are not presently any thermal barrier coatings in production on steel pistons.

In the most recent DOE annual merit review meeting, Cummins, Daimler, Volvo, and PACCAR all reported that they are studying use of thermal barrier coatings in their SuperTruck II projects. At that same meeting, others (e.g., Ford) reported they are studying such coatings for light-duty applications.¹⁰

Potentially, a next-generation thermal barrier coating could be based on an aerogel, a technology that was used to manage heat on the space shuttle upon reentry. Aerogel composites have also been used in aviation interiors where lightweight is critical.¹¹ An aerogel is a synthetic porous material derived by extracting the liquid component of a gel through supercritical drying. With most of the volume being air (or vacuum), the resulting solid has extremely low thermal conductivity. Some initial experiments using an aerogel as a thermal barrier coating were unsuccessful due to adhesion problems, which could be solved with further materials development and surface engineering.

Ceramic thermal barrier coatings are already commonly used on production aviation turbo-shaft engines where extremely high temperatures are encountered on both moving and stationary components. Unlike the case for internal combustion engines where high temperatures during the

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⁹ Toyota, 2015, “Toyota’s Revamped Turbo Diesel Engines Offer More Torque, Greater Efficiency and Lower Emissions.” https://global.toyota/en/detail/8348091.

¹⁰ See the 2020 “Annual Merit Review Presentations” at U.S. Department of Energy Vehicle Technologies Office website at https://www.energy.gov/eere/vehicles/annual-meritreview-presentations.

¹¹ Aerogel Technologies, “Markets and Technology,” http://www.aerogeltechnologies.com/applications/, accessed November 2020.

intake stroke compromise volumetric efficiency, a low specific heat capacity is not needed for parts coated on gas turbines.¹²

POWER ELECTRONICS OPPORTUNITIES AND CHALLENGES

In their raw form, almost all electrical energy sources today are incompatible with the loads they are supplying. The parameters of supply—for example, voltage, frequency, current—must be converted to those required by the load. Examples are a solar array producing variable DC voltage supplying an AC grid of constant frequency and voltage, or batteries producing DC power in a hybrid vehicle to supply motors requiring variable AC voltage and frequency, or even a battery whose voltage varies with use to power a radio requiring constant voltage. The interface in these energy systems consists of electronic devices configured to provide the necessary transformations. Such an interface is known as a power-electronics converter and will be ubiquitous in the Army’s power and energy technologies of the future. These converters add volume and weight to the battlefield equipment inventory. To a large extent, the volume and weight are dictated by the thermal management requirements because the converters are not 100 percent efficient. Newly developed semiconductor devices using the wide band-gap materials silicon carbide (SiC) and gallium nitride (GaN) promise to improve the thermal performance of future power electronic converters.

Because thermal management plays such a critical role in all ground combat vehicles, technical electrification challenges in power density and temperature threshold have been identified by the Army as part of its hybrid studies. Running electronics at higher temperatures, preferably using coolant at the same temperature of the internal combustion engine, reduces cooling system losses. The Army’s “wants” for power electronics use are summarized in Table 8.1.

The current challenge using SiC and GaN is that the size of wafers of the necessary purity and freedom from defects limits the power that transistors made of these materials can control. The development of SiC as a semiconductor device material was done by Cree with partial funding from the Defense Advanced Research Projects Agency (DARPA) and the State of New York.¹³ While SiC devices are fabricated on native substrate, GaN devices are produced in an epitaxial layer on a substrate

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¹² S.M. Meier and D.K. Gupta, 1994, The evolution of thermal barrier coatings in gas turbine engine applications, Journal of Engineering for Gas Turbines and Power 116(1):250–257.

¹³ Cree, Inc., 2019, “Cree & NY CREATES Announce First Silicon Carbide Wafer Demonstration at SUNY Poly in Albany,” https://www.cree.com/news-events/news/article/cree-ny-creates-announce-first-silicon-carbide-wafer-demonstration-at-suny-poly-in-albany.

TABLE 8.1 U.S. Army Power Electronics Goals

SOURCE: K. Boice, 2020, “Combat Vehicle Energy Storage,” SAE Hybrid and Electric Vehicle Technologies Symposium, January 28.

of Si, SiC, or Al₂O₃ (sapphire). The disparate physical properties of the two materials—for example, thermal coefficient of expansion—produces a challenge at the interface of the epitaxy and substrate, resulting in suboptimal device behavior. A further important constraint imposed by devices fabricated on an epitaxial layer is that their geometry has to be lateral, which is real-estate intensive. Power devices are almost universally vertical structures, meaning the current flows vertically through the substrate, providing the necessary length to support high voltage without sacrificing surface area. Research on using native GaN is proceeding, and success will be necessary before GaN device geometries can be vertical and useful in power applications.

The two most important parameters that provide SiC’s advantage over Si are its thermal conductivity and critical electric field E_c, the field at which the material breaks down. As Table L.1 in Appendix L shows, SiC has more than three times the thermal conductivity, and nearly a 10-fold increase in E_c of Si. The benefits of increased thermal conductivity are clear. The increase in the critical field permits a much thinner device to support a given voltage, which reduces both the thermal resistance and on-state voltage drop of the transistor.

The Army’s goals for volumetric and gravimetric parameters of energy-conversion apparatus (e.g., electric vehicle or hybrid traction drives) suggest designs at higher electrical frequencies. From a power-electronic perspective, higher frequencies result in smaller energy-storage components. These components comprise the principal sources of weight and volume. Capacitors and inductors form necessary filters, transformers provide required scaling of voltage, and electrical machines (motors and generators, which are essentially electrical to mechanical transformers) provide the required physical work.

A further factor influencing the gravimetric and volumetric specifications of power electronic systems is the thermal limitations of their components. Silicon carbide has made possible semiconductor devices with maximum junction temperatures exceeding 200°C, while Si transistors are generally limited to a junction temperature of 175°C. The thermal limits of current packaging technology prevent fully exploiting the higher thermal ratings of SiC. This increased upper temperature limit combined

with the very high thermal conductivity of SiC compared to Si reduces the size of the thermal-management hardware for cooling the device. However, the passive components, especially capacitors, with compatible thermal ratings have not yet been developed. So, to truly reduce the size and weight of power electronics, magnetic and dielectric materials with higher thermal ratings need to be developed.

Additional background material on the power electronics challenge and how they can be addressed is contained in Appendix L.

ALUMINUM METAL MATRIX COMPOSITE (MMC) APPLICATIONS

Of growing importance, metal matrix composites (MMCs) are emerging as high-performance alternatives to traditional alloys. MMCs consist of two or more constituent parts, one being a metal and the other another material, such as a ceramic or organic compound dispersed throughout the metal matrix. For example, ultrafine particles of SiC are commonly used in an aluminum matrix to improve its material properties.

This reinforcement can serve a purely structural task, such as greater strength-to-weight, higher yield point and ultimate tensile strength, improved strength-to-weight ratio, and greater fatigue strength at elevated temperatures. In addition, the selected reinforcement can be used to change physical properties, such as providing a lower thermal expansion, lower friction coefficient, greater wear resistance, greater thermal conductivity, improved coefficient of thermal expansion, improved elastic modulus, and/or improved machinability or near-net-shape forgeability versus conventional engine materials.

The Army is presently conducting extensive studies of aluminum MMCs. This area of investigation will enable improved structural properties in a lighter-weight format. Advances would be important for major engine components in specific applications, such as engine blocks and cylinder heads for unmanned aircraft systems.

Application of aluminum MMCs needs to be compared with other material alternatives, such as magnesium and titanium. The Army’s needs may deviate a significant amount from those of commercial OEMs because

cost may play a less significant role, particularly in weight-sensitive applications such as unmanned aerial vehicles (UAVs).

UNIQUE METAL MATRIX COMPOSITE MATERIALS FOR PISTONS

Most modern military diesel engines use steel pistons based on their ability to tolerate higher temperatures and higher peak cylinder pressures. The yield and fatigue strength of aluminum is typically inadequate for diesel engine peak–cylinder pressures above 200 bar (also dependent on the piston compression height) and begins to fall off sharply at temperatures above 300°C.

Besides higher strength at high temperatures, another advantage of steel-piston use is their similar coefficient of thermal expansion to iron. When used with a grey iron or compacted graphite iron block, tighter piston-to-bore clearances are enabled. In contrast, aluminum, with its roughly three times greater coefficient of friction at rated power, often exceeds the bore and runs in a compressed mode within an iron block cylinder bore.

Aluminum MMC pistons may not be capable of standing up to the high piston crown temperatures and cylinder pressures of an opposed piston engine. However, titanium, with its higher melting point (about 1,000°C above aluminum) and comparable strength properties to steel may play a role in developing a suitable MMC piston material. Within industry, there has already been some work with titanium MMCs.

Titanium has higher tensile strength than steel but is not presently used in pistons because of its poor thermal conductivity. Although it can tolerate much higher temperatures, its inability to dissipate the heat of combustion can result in excessively hot crown temperatures leading to premature ignition and engine damage. This thermal conductivity would need to be increased with the addition of the second matrix component, perhaps some form of elemental carbon.

ARTIFICIAL INTELLIGENCE/MACHINE LEARNING-BASED MATERIAL OPTIMIZATION

Numerous studies have demonstrated the benefits of using artificial intelligence (AI)/machine learning (ML) to quickly evaluate the plethora of design options for improved material properties. Included among these are studies of various metallic alloys.¹⁴

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¹⁴ J. Wei, X. Chu, X.-Y. Sun, K. Xu, H.-X. Deng, J. Chen, Z. Wei, and M. Lei, 2019, Machine learning in materials science, InfoMat 1(3):338–358, doi: 10.1002/inf2.12028.

As a research project, combining the following efforts with AI/ML materials studies may provide some significant benefits:

• MMC pistons and conrods (connecting rods);
• Ceramic matrix composites (CMC) for high-temperature components, such as exhaust manifolds;
• Compatible liner materials or block materials (if a parent metal block) with piston skirt and rings;
• Possible unique skirt materials or coatings—possibly diamond like coating or higher temperature–capable polymer base coating;
• Thermal barrier coatings—matched to adhere to the MMC crown material and minimize heat transfer needed to undercrown—possibly used selectively on the outside of a liner to allow more uniform temperatures within the bore; and
• AI/ML algorithms to enable further exploration of the materials design space without relying exclusively on testing.

Such new piston materials and architecture may provide lower reciprocating mass, enabling higher speeds and increased power at equal peak cylinder pressures. Furthermore, reduced thermal expansion would enable tighter piston-to-bore crevice volumes, thereby improving power density and fuel efficiency.

3D PRINTING/ADDITIVE MANUFACTURING

Three-dimensional (3D) printing, also known as additive manufacturing, is a process for making a physical object from a 3D digital model, typically by laying down many successive thin two-dimensional layers of a material. It brings a digital object (its computer-aided design [CAD] representation) into its physical form by adding layer by layer. As such, it enables geometries not previously possible, plus by making it possible to eliminate joints, it increases the reliability of the product while reducing size and weight. In addition, 3D printing can accelerate design and testing of prototypes, thereby shortening the development period.

The earliest 3D printing process fabricated 3D plastic models using a photo-hardening thermoset polymer. Each layer would then be exposed to the appropriate ultraviolet (UV) beam to harden selected areas. Since that time, there have been a wide variety of improvements in 3D printing materials. Initially, plastic engine intake manifolds produced by 3D printing were not capable of withstanding high pressures associated with turbocharged engines. However, with improved plastic materials, that is now possible.

3D printing with a variety of metals has been demonstrated, including intake manifolds in aluminum. IAV, the German consulting firm, has

proven that prototype steel pistons can be fabricated using 3D printing to quickly explore different engine combustion regimes.¹⁵ SpaceX is making rocket components using 3D printing.¹⁶ Pratt & Whitney will be the first to use additive machining technology to produce compressor stators and synch ring brackets for their production turbine engines.¹⁷ 3D printing of titanium aerospace components is now available.

Chemnitz University of Technology in Germany recently showcased an electric motor produced entirely by additive manufacturing. Highly viscous metallic and ceramic pastes were extruded through a nozzle to build the body of the parts in layers. This assembly was then sintered to the required harness. They designated this process “multimedia 3D printing.”¹⁸

A key advantage of 3D printing is the ability to eliminate joints that are difficult to produce with more traditional casting and machining methods, thereby reducing cost, schedule time, and weight and improving reliability. The automotive industry has spent much effort using this and other innovative design approaches to eliminate joints. One joint-elimination example (not created with 3D printing) is the integrated exhaust-manifold cylinder head used in production by GM, where both the cylinder head and exhaust manifold are part of a common casting. Another approach used by Honda in their GC-family engines, called monobloc construction, is the integration of the cylinder head and block to eliminate the need for head gaskets, a high warranty item.¹⁹

Costs associated with 3D printing versus other manufacturing methods have precluded its widespread adoption in the past. It is often used for low-volume prototype parts that are needed quickly or have significant tooling costs with traditional manufacturing methods, such as casting and machining. However, 3D-printing costs have come down quickly, to the extent that 3D printing is now routinely used for higher volume production, such as the cores for precise cooling passages within a cylinder-head casting. Interestingly, Porsche now uses 3D-printed pistons

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¹⁵ K. Buchholz, 2018, “IAV Using 3D Printed Pistons for Engine Testing,” SAE International, https://www.sae.org/news/2018/04/iav-using-3d-printed-pistons-for-engine-testing.

¹⁶ B. Salmi, 2019, “The World’s Largest 3D Metal Printer Is Churning Out Rockets,” IEEE Spectrum, October 25, https://spectrum.ieee.org/aerospace/space-flight/the-worldslargest-3d-metal-printer-is-churning-out-rockets.

¹⁷ Aerospace Manufacturing and Design, 2015, “Pratt & Whitney AM Engine Parts Poised for Entry into Service,” https://www.aerospacemanufacturinganddesign.com/article/pratt-whitney-additive-parts-engine-040615/.

¹⁸ M. Fejes, 2018, “Premiere at Hannover Messe: Fully 3D-Printed Electric Motors,” Chemnitz University of Technology, https://www.tu-chemnitz.de/tu/pressestelle/aktuell/8718/en.

¹⁹ Precise Equipment Repair, 2017, “Honda General Purpose Engines,” https://web.archive.org/web/20101127185645/http://perr.com/honda.html.

produced by Mahle in its 911 GT2 RS, one of its higher-performance production vehicles.²⁰

FUEL CELL MATERIALS

Solid Oxide Fuel Cells

The materials for the main components of a solid oxide fuel cell (SOFC) have been reviewed and discussed extensively in the literature.^21,22 The most commonly used electrolyte material in SOFCs is zirconia stabilized with either Y₂O₃ (YSZ) or Sc₂O₃ (ScSZ); SOFCs using these electrolytes need to be operated above about 800°C to achieve sufficient ionic conductivity. Alternate electrolyte materials have been developed for lowering the SOFC operating temperature down to about 550°C; these include stabilized bismuth oxide (Bi₂O₃) and ceria (CeO₂). However, stabilized Bi₂O₃ is easily reduced and decomposes to bismuth metal, and doped ceria develops electronic conductivity under the low-oxygen partial pressures of the fuel; therefore, these materials need to be protected on the fuel electrode side with a protective coating (such as YSZ or ScSZ) for their successful use as the electrolyte. Doped perovskites such as lanthanum gallates, barium cerates, and strontium zirconates have also been investigated for use as intermediate temperature (600–800°C) electrolytes with some success. The Army has shown an interest in lowering the operating temperature of SOFCs to 300–600°C for certain applications compared to 700°C or higher of currently available SOFCs and has recently requested Small Business Technology Transfer solicitations for such work.²³ Proton-conducting perovskite electrolyte materials such as BaCo_0.4Fe_0.4Zr_0.1Y_0.1O₃₋δ, NdBa_0.5Sr_0.5Co_1.5Fe_0.5O₅₊δ, and PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ offer an opportunity to develop small SOFC systems capable of running on hydrocarbon fuels such as propane and operating at 300–600°C. However, such proton-conducting electrolytes

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²⁰ Porsche Newsroom, 2020, “Innovative Pistons from a 3D Printer for Increased Power and Efficiency,” https://newsroom.porsche.com/en/2020/technology/porsche-cooperationmahle-trumpf-pistons-3d-printer-power-efficiency-911-gt2-rs-21462.html.

²¹ S.C. Singhal, 2001, “Zirconia Electrolyte-based Solid Oxide Fuel Cells,” pp. 9898–9902 in Encyclopedia of Materials: Science and Technology (Second Edition), https://doi.org/10.1016/B0-08-043152-6/01792-7.

²² S.C. Singhal and K. Kendall, 2003, High-Temperature Solid Oxide Fuel Cells: Fundamentals, Design, and Applications, Amsterdam: Elsevier Publishing, https://www.elsevier.com/books/high-temperature-solid-oxide-fuel-cells-fundamentals-design-and-applications/singhal/978-1-85617-387-2.

²³ U.S. Army, 2020, “300W Low-Temperature SOFC Army Power Sources,” STTR Solicitation A20B-T003, https://www.sbir.gov/node/1696401.

suffer from chemical stability issues in CO₂ and H₂O that is formed on the SOFC anode side, which need to be addressed and resolved.

The most widely used material for SOFC anodes is a cermet of Ni with YSZ or doped ceria. Other anode materials under investigation include perovskite structure conducting ceramics, such as suitably modified strontium titanate. Nickel is easily poisoned by sulfur in the fuel requiring desulfurization of all SOFC fuels to a sulfur level below about 1 ppm. For the use of diesel and JP8 fuels by the Army, it is desirable to find anode materials with higher sulfur tolerance; adding certain dopants to nickel-based anodes such as CeO₂ or using conducting ceramics may provide better sulfur tolerance than nickel.

The high operating temperature of SOFCs allows the use of only noble metals or electronic conducting oxides as cathode materials. However, the high cost of noble metals such as platinum or palladium prohibits their use in practical SOFCs. Doped lanthanum manganite (such as LSM) and doped lanthanum ferrite (such as LSCF) are most commonly used for SOFC cathodes. Other possible cathode materials include perovskite-structured oxides such as lanthanum cobaltite and lanthanum nickelates, suitably doped with alkali and alkaline earth ions to tailor the conductivity and thermal expansion coefficient. Selection and development of a suitable cathode material capable of providing high cell performance and performance stability with time is important in developing high power density and lower-cost SOFCs.

The choice of the interconnect material depends on the cell operating temperature. For cells operating at about 900–1,000°C, alkaline earth-doped lanthanum chromite (LaCrO₃) is used for the SOFC cathode. However, this ceramic material is expensive and difficult to sinter. Therefore, in cells operating at 700–800°C, cheaper metallic interconnects, such as high Cr-content stainless steels, are used. However, chromium volatilization from these metallic interconnects tends to degrade the cell performance and therefore these interconnects require protective ceramic coatings to reduce chromium vaporization. Research is continuing to identify, develop, and optimize such protective coatings.

Proton-Exchange Membrane Fuel Cells

DOE has sponsored much of the work on proton exchange membrane (PEM) fuel cells and has described the basic materials used for the various cell components.²⁴ Central to a PEM fuel cell is the membrane electrode assembly (MEA), which includes the membrane (the proton

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²⁴ U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, “Parts of a Fuel Cell,” https://www.energy.gov/eere/fuelcells/parts-fuel-cell.

conducting electrolyte), the two electrode layers (with catalysts), and the gas diffusion layers (GDLs). For low-temperature PEM fuel cells for operation from about 60°C to 90°C, the electrolyte membrane is generally a fully fluorinated polymer (such as Nafion manufactured by DuPont). For high-temperature PEM fuel cells for temperatures up to about 120°C, a polybenzimidazole (PBI) doped in phosphoric acid is generally used as the electrolyte. The electrolyte membrane is usually very thin, as thin as 20 microns. Anode and cathode catalyst layers are added to the two sides of the electrolyte membrane; conventional catalyst layers include nanometer-sized particles of platinum dispersed on a high-surface-area carbon support (Figure 8.4). The gas diffusion layer (GDL) typically consists of a sheet of carbon paper in which the carbon fibers are partially coated with polytetrafluoroethylene (PTFE); GDL facilitates transport of reactants into the catalyst layer and the removal of product water.

Continuing research and advancements are needed to reduce cost and improve performance and durability of PEM fuel cells. Platinum catalyst

is a major cost of the cell; catalysts with reduced or no platinum group metal, increased activity and durability, and lower cost should be investigated. To reduce degradation, catalyst supports with increased durability and conductivity should also be investigated. To improve PEM fuel cell durability, research and development should focus on understanding the fuel cell degradation mechanisms and developing materials and strategies to overcome them. In addition, the practicality of on-board reformation of hydrocarbon fuels to produce CO- and S-free hydrogen for PEM fuel cells for mobile ground and air vehicles should be investigated.

TEMPERATURE AND RADIATION-RESISTANT MATERIALS FOR NUCLEAR REACTORS

A fundamental driving force in nuclear power is to have a higher safety margin in case of a reactor accident. There are three materials concepts with the goal to mitigate the negative zircaloy interactions with hot steam at high temperatures, or to eliminate this chemical reaction altogether. One of the materials concepts has the potential to advance the Army effort on developing a safe micro-nuclear reactor (MNR) for military installations based on gas coolants, one of which is an inherently non-reactive gas, helium.

The most common and seemingly most straightforward solution for current light-water reactors is to coat the zircaloy cladding with a material that is resistant to oxidation with steam that produces dangerously explosive hydrogen gas. Over the last 8 years, chromium coating has gained a consensus in the nuclear reactor fuel community as being the most straightforward to deploy based on its stage of development and testing to date. It is a near-term option to improve current light-water reactor safety. Chromium-coated fuel rods were inserted into the Illinois Byron reactor in September 2019 to accumulate irradiation testing data and show that potential changes in the coefficient of thermal expansion, or chemical adhesion, do not cause the coating to delaminate under normal operating conditions. The final results of this testing effort are still pending; however, there still remains the question of how this coating will perform during a loss of coolant accident.

A second option is to use an alloy composed of iron, chromium, and aluminum. This metal can be extruded in the same way that zircaloy is and does not require the extra steps for coating. It also eliminates the problematic metal, zircaloy, that has such deleterious effects in accident conditions. However, this alloy introduces a significant penalty by absorbing neutrons, causing the fission process to generate less heat that can be converted into electricity. This problem would require additional fuel enrichment at a substantial cost to produce additional neutrons. Either

way, there will be a negative effect on the economics of nuclear-generated electricity using this cladding. Although testing on this alloy started in February 2018 in a commercial reactor (Hatch-1 in Georgia) with no fuel, the neutron penalty makes it unlikely that it will eventually be a commercial product.

The third materials concept and the most promising option has been to replace the metal cladding with a pseudo-ductile ceramic. A unique ceramic material, SiC, has been in testing for decades and was first recognized by the fusion research community to be resistant to neutron damage. This robust material was already used in the nuclear industry to make the shell structure of Tri-structural Isotropic (TRISO) fuel particles. Each TRISO particle is made up of a uranium, carbon, and oxygen fuel kernel. The kernel is encapsulated by three layers of carbon- and ceramic-based materials that prevent the release of radioactive fission products. This shell structure is about 30-microns thick and has been extensively studied by Idaho National Laboratory, Oak Ridge National Laboratory, and Los Alamos National Laboratory. TRISO fuel is the current choice for the reactor designs to power Army MNRs.

Today, SiC can also be made into 10-micron diameter fibers in bulk quantity. In addition, SiC is used in the semiconductor industry as well as in the aerospace industry because of its temperature-resistant properties.

On the technology development side, the SiC fibers and SiC material can now be combined to make a ceramic fuel rod by having the fibers embedded in the bulk SiC material. This novel material is called SiC composite or SiC-SiC. The fundamental properties of the material enable resistance to high-temperature, high-stress, and high-neutron flux. In contrast, most metals tend to soften and lose their strength at temperatures above 700–800°C. Also, the metal in current reactors degrades as a result of neutron damage, which limits its lifetime. In addition, as mentioned earlier, zircoloy cladding has the deleterious thermal runaway reactions with steam that produce hydrogen gas and reactor core meltdowns. SiC-SiC would effectively eliminate all these problems because it does not disassociate until about 2,700°C. It also retains its strength to a temperature of 1,700°C in prototypical nuclear reactor accident conditions. It holds its shape during accident conditions through its reinforcing fibers, which act structurally like rebar in cement. Samples already tested in the high-flux isotope reactor (HFIR) at Oak Ridge National Laboratory show resilient material properties.²⁵

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²⁵ K. Linton, 2020, “Scientists Building 3D-Printed Nuclear Reactor Core Use HFIR to Test Novel Materials,” Oak Ridge National Laboratory, https://www.ornl.gov/news/scientistsbuilding-3d-printed-nuclear-reactor-core-use-hfir-test-novel-materials.

TRISO fuel kernels are 200–500 microns in diameter, where the SiC shell serves as a tiny pressure vessel to retain fission gases that are the potentially dangerous emissions from a serious reactor accident. However, SiC-SiC composite matrix technologies can now be used to make fission gas leak-proof fuel rods and loaded in a similar fashion into light-water reactors as a standard fuel. This unique composite material is scheduled for insertion into the Idaho National Laboratory Advanced Test Reactor (ATR) in 2023.

SiC-SiC technology also applies to gas reactors, a coolant of choice for Army MNRs, that are typically designed to use costly TRISO fuel. However, using SiC-SiC fuel-rod elements can reduce the precious volume in the reactor core that is lost by using TRISO fuel. This replacement can enable higher power densities and increased electric power generation without a weight and volume penalty while maintaining safety.

Other Material Considerations

Many technologies and systems of interest for the Army rely on critical materials. In general, it is better to develop technologies or systems that do not rely heavily on raw materials that are sourced outside the United States. Supply-chain issues can cause significant national security and economic implications for the country. As examples, ensuring sufficient availability of both lithium and cobalt for military electrification are potential concerns.^27,28 A recent evaluation of supply-chain risk versus natural abundance of battery-relevant elements buttressed this concern.²⁹

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²⁶ See Appendix M and Chapter 7 for additional information.

²⁷ T. Paraskova, 2020, “A Major Supply Shortage Is Set to Hit Lithium Markets,” https://oilprice.com/Energy/Energy-General/A-Major-Supply-Shortage-Is-Set-To-Hit-Lithium-Markets.html.

²⁸ N. Kobie, 2020, “As Electric Car Sales Soar, the Industry Faces a Cobalt Crisis,” Wired, https://www.wired.co.uk/article/cobalt-battery-evs-shortage.

²⁹ B.J. Hopkins, C.N. Chervin, M.B. Sassin, J.W. Long, D.R. Rolison, and J.F. Parker, 2020, Low-cost green synthesis of zinc sponge for rechargeable, sustainable batteries, Sustainable Energy Fuels 4:3363–3369.